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FIRST DEGREE HEART BLOCK
Introduction
First Degree Heart (or atrioventricular) Block is (somewhat arbitrarily) defined as a PR interval on ECG of greater than 0.20 seconds, (or 200 milliseconds).
The nomenclature is actually misleading, as the pathology is a delay in conduction rather
than a total “block”. 1
The abnormality of itself is clinically benign.
It may be a normal finding in some people such as athletes, however it may also indicate
an underlying cardiac abnormality that may or may not require attention.
The clinical setting is therefore important when determining whether any particular
treatment, monitoring or further investigation will be required in any given case.
Pathology
Conduction delay
The P-R interval represents the time needed for an electrical impulse from the sinoatrial
node to conduct through the atria, the AV node, and the His bundle - Purkinje system
fibers. The majority of delay occurs at the AV node. 1
Thus, as shown in electrophysiologic studies, P-R interval prolongation (i.e., first-degree
AV block) may be due to conduction delay within the right atrium, the AV node, the HisPurkinje system, or any combination of these.
Overall, dysfunction at the AV node is far more common than dysfunction at the HisPurkinje system. If the QRS complex is of normal morphology on the ECG, then the
conduction delay is almost always at the level of the AV node. If, however, the QRS
demonstrates a bundle-branch morphology, then the level of the conduction delay is often
localized to the His-Purkinje system.
The abnormality may be seen in isolation or in association with other blocks (e.g., LBBB,
RBBB, second-degree AV block, and trifascicular block).
Causes:
The causes of a first degree heart block include:
1.
2.
Physiological:
●
Normal variant
●
Athletes, (increased vagal tone)
●
Increased vagal tone in general (from any cause).
Electrolyte disturbances:
●
Potassium disturbances:
♥
●
3.
Interestingly both hyperkalemia and hypokalemia can result in an
increased P-R interval.
Hypermagnesemia
Drugs:
First degree heart block may be seen in the setting of acute drug overdose as well
as that of chronic toxicity.
It may be seen in particular with:
●
Digoxin
●
Class Ia Antiarrhythmic agents
●
♥
Quinidine
♥
Procainamide
Class Ic antiarrhythmics:
♥
●
Class II Antiarrhythmic agents
♥
●
●
Flecainide
Beta blockers
Class III Antiarrhythmic agents
♥
Amiodarone
♥
Sotalol
Class IV Antiarrhythmic agents
♥
4.
5.
Calcium channel blockers
Myocardial Ischemia:
●
ACS, (inferior ischemia in particular)
●
Conduction disturbance from chronic ischaemic cardiomyopathy, which
can include intrinsic disease of the AV node.
Myocarditis, (of any cause):
●
One important consideration in this group is the possibility of Rheumatic
Fever.
6.
Cardiomyopathies, (of any cause)
7.
Valve lesions:
●
Mitral or aortic valve pathology or surgery.
Clinical assessment
As first degree heart block may be a normal variant in some people, or may be present in
trained athletes, no further assessment may be necessary.
The clinical setting is therefore important, and a history and examination should be based
on the known causes of first degree heart block
The presence of ACS is especially important, as will be a history of the patient’s
medications, including the possibility of deliberate overdose.
The possibility of Rheumatic Fever, where first degree heart block constitutes one of the
minor criteria for the condition, should be kept in mind, in those who are in high risk
groups for this disease.
Investigation
Blood tests
None may be necessary, depending on the clinical scenario, however in general the
following should be considered:
1.
FBE
2.
U&Es/ glucose
●
In particular serum potassium
3.
Serum digoxin level
4.
Serum magnesium
5.
Troponin:
●
In the setting of actual or suspected cardiac disease
ECG
The normal PR interval is 0.12 to 0.20 seconds, (or 3-5 small squares)
Note that although conduction is slowed, there are no missed ventricular beats.
Standard tracing speed and ECG grid spacing:
ECG grid makings 1
Rhythm strip showing first degree heart block. The P-R interval is greater than 1 large
square.
Rhythm strip showing severe first degree heart block. The P-R interval is beyond 300
milliseconds and the P wave almost merges with the preceding T wave.
Management
First degree heart block, of itself usually requires no specific treatment, whether the cause
is benign or not.
The task will be to assess whether or not there is an underlying pathology which may
require urgent treatment - for other reasons.
Drugs which cause conduction delay should be avoided or only used with caution.
Patients with an ACS, myocarditis, or acute drug overdoses are at risk of progression to
higher degrees of block and so must be observed closely.
While asymptomatic first-degree AV block does not require treatment, patients with
severe bradycardia or those with the possibility of progression to higher-degree AV
block, (such as beta blocker overdose) drug treatment (e.g., atropine or adrenaline) may
be used as a temporizing measure to the placement of a cardiac pacemaker.
References
1.
Conover M. Understanding Electrocardiography, 7th ed, 1996.
Dr J. Hayes
Reviewed September 2012.